Low temperature vitreous coatings for steel



United States Patent O LOW TEMPERATURE VITREOUS COATINGS FOR STEEL Edward J. Feeney and Ernest M. Hornmel, Pittsburgh, Pa., assignors to The 0. Hommel Company, Pittsburgh, Pa., a corporation of. Pennsylvania 7 No Drawing. Application May 28, 1954 Serial No. 433,270

19 Claims, (Cl. 117-40).

This invention relates. to low temperature vitreous coating of steel. More particularly the invention relates to improved vitreous coatings which may be applied at low temperatures while avoiding the scaling, thewarping, the formation of gas bubbles and the necessity of coating thin sheet steel on both sides.

A large number of household appliances and building materials made of comparatively thin sheet steel are now in use which have vitreous enamel coatings. The enamel coatings are fused on the sheet steel at comparatively high temperatures, i. e. from 1400 to l700 F. This high} temperature heating of sheet steel encounters many prob; lems which cause many coated plates to be rejected on account of imperfections. When thin sheet steel is heated at temperatures of 1300 or above the steel sags and warps and excessive oxidation-occurs. This oxidation will take place on a bare sheet or under the enamel of a sheet of steel coated with a slip or enameling-compound. At these high temperatures 4 to 5-mils are-required in order to provide a glass-that will dissolve or absorb the iron oxide being formed Further this amount of enamel is generally applied to both sides of the steel sheet to keep it from warping. 7

When sheet steel is heated between 1200 and 1300- F. gaseous hydrogen is set free from the sheet surface. Also moisture in the slip and on the steel react with carbon in the steel when heated at temperatures of 1200 to 1500 F. to produce carbon gases which may. generate bubbles in the enamel coating.

If sheet steel is heated in the temperature range of 850 to 1100 F. substantially nosag'ging, warping or cles such as steel, sash and window frames as made have hollow parts which cannot be, coated inside- It is im portant therefore to be able to coat. such articles onthe outside only without warpage. of steel which cannot be satisfactorily coated.;with'vitre.- ous enamel at temperatures of 1400 to 1700 Ft can. be satisfactorily coated by the process and. vitreous enamel of the present invention.

We have found that excellent hard, uniform. enamel. coatings which have goodbonds or adherence-may be,

obtained when the enameling operation is. carried out at. temperatures of 850 to 1100 F. At these. low ,tem-

peratures the enameled sheets maybe bent without chipping and coatings may be obtained which are'acid, alkali, and weather-resistant. Further the low tempera: ture enameling operation permits. a bettercontrolof-the character of the surfacegloss or. lack ofgloss; than high temperature enamelingprocesa,

Most of' the chemi-.

In fact. very thin sheetsthe.

These improved results areobtained by new enameling. compounds or slips which may be.made in any desiredcoloring and which are flexible and adhere so well to the steel. that they do not chip off; i

The primary object of the present invention is to provide a vitreous enamel suitable for producing a fused enamel coating on ferrous alloysat temperatures below 1200 F.

Another object of the invention is to provide a process, by which ferrous metal alloys having a coeflicient of expansion in the range of 12 to 14x10 may be given a fused enamel coating when treated at temperatures in the range of 850 to 1100" R, which temperatures are much lower than the temperatures now being used by-the' trade for enameling steel.

A further object of the invention is to provide a process of coating sheet steel articles with a fused vitreous enamel at temperatures below1200 F. to provide a coated article which is alkali, acid and weather-resistant.

A further object of the invention is to provide a stable enameling composition which can be prepared ready for application to steel such that it may be retained in its prepared state for a long period of time and be satisfactory for use.

With these and other objects in view the invention con-. sists in the new enameling composition and the process. of enameling steel wares with the composition as herein: after described and particularly defined in the appendedclaims,

The present invention has a large application for. the purposeof coating ferrous alloys such as 'steel, cast iron, Wrought iron, and the many forms of stainless steelwhich contain more than 20% of iron. As outlinedaboye, it is important to be able to put acontinuous vitreousv enamelcoating on various metals, particularly thin ferrous metals.

In carrying out the present invention, two types offrits are used. One frit is preferably used for coating the raw metal, and'another fritof comparable composition is used for the cover coat. The compositions of-these two types of fritshave been carefully worked outv and tested to obtain glass frits which have thermal linear coefficients of expansion within 1x10" of the coetficients of expansion of the ferrous alloys to be coated, and particularly with sheet steels which are extensively used in the manufacture of appliances such as refrigerators, stoves, and the like.

Seven ground coat frits are shown in Table I.

GROUND COAT FRITS.PERCENTAGE BY -WEIGHT Table I This same series of groundcoat frits are shownj in, Table II in which-the-parts byweight are expressed in P at ss t.

. 7 3 I MOL. PERCENT Table II a b c d e f g By reference to Table II it will be seen that the range of variation of the constituents in the various ground coat frits is shown in Table III.

GROUND COATS MOL PERCENTAGE Table III N820 13.9 to 17.3 K 0.4 to 3.0 L120 0.0 to 6.0 13,0 7.5 to 13.1 810, 33.4 to 38.7 CaO 1.0 to 15.6 CnO 0.0 to 3.2 MgO 0.0 to 13.5 TiO 0.0 to 4.2 P150 0.0 to 14.0 A1203 0.0 to 0.6 F 0.0 to 8.3 Pbo' 0.0 to 14.0 BaO 0.0 to 5.2 ZnO 0.0 to 3.0 P205 0.0 to 0.5 NiO 0.0 to 1.6 C0304 0.0 to 0.2

In Table IV are shown the compositions of four difierent frits which are suitable for use as cover coats.

CALCULATED MELTED COMPOSITIONS FOR COVER COAT FRITS.WEIGHT PERCENT Table IV A B O D The compositions shown in Table IV are expressed in mol percentages in Table V.

MOL PERCENT To coat sheet steel for example with the frits of Tables II, III and V, it is important that the steel should be carefully cleaned before the frits are applied to the steel. The cleaning consists in boiling the steel in a cleaner which may be one of a large number of proprietary cleaners now on the market such as Oakite and Wyandotte. This boiling treatment removes grease and dirt. After the boiling treatment the steel is rinsed with water and then given a pickling treatment with 6 percent sulphuric acid and heated to a temperature of about 140 F. The pickled steel is then rinsed with water and put into a nickel dip which is a solution of nickel sulphate for the purpose of depositing a thin film of nickel on the surface of the steel. The nickel-dipped steel is then rinsed again with water to complete the cleaning operation and is neutralized to remove acid and then dried. To the dried and cleaned sheet steel then is applied a surface coating of a frit in the form of a slip.

Many metal articles, especially the thicker ferrous metals, are preferably cleaned by sand blasting being coated with nickel before having the slips applied thereto.

In Table VI are shown three examples of ground coat enamel slips.

GROUND COAT ENAMELS.PARTS BY WEIGHT Table VI Frit (a). 2 Sodium silicate (N-brand). 1 Borax. 30' Water.

16% frit (f). 100 33 /3 frit (d).

[50 frit (b). V2 Refined bentonite.

V2 Urea.

30 Water. 4 Methanol.

100 Parts frit (a). 11.1 Parts titanium dioxide.

25 Parts water. 4 Parts methanol.

These frits are prepared by a smelting operation in which the dry ingredients shown in Table I are mixed together and then heated in a furnace at a temperature of 1800" to 2100 F. to melt the ingredients and then the molten materials are discharged into a body of water or passed through water-cooled rolls to solidify the frits in the form of granular glass. The granular glass frits are then treated to form the slips shown in Table VI by grinding in a water or vehicle suspension to a finely ground material. The grinding is preferably carried out in a ball mill. The fluids, such as water and methanol and sodium silicate, are added to the ingredients as part of the milling charge of the ball mill.

Also coloring pigments such as cobalt-chrome-iron, black cobalt oxide, nickel oxide, iron oxide, copper oxide, and opacifiers, such as titanium dioxide and zirconium oxide to the extent of 0.5 to 10.0% by weight of the frit are added to the frits as they are being prepared as slips. The-preparation of the Slips provides a suspension of the oxides in water, alcohol, and silicates to have a consistency suchthat the slip may be applied to the steel plates or articles with a spray gun to obtain a coating by spraying. Furthermore the slip may be painted on the steel plate, or the plate may be coated by dipping. After the coat has been applied to cover the entire surface which is to be enameled, the object is placed in a dryer at about ZOO-250 F. to remove all volatiles and then placed in a furnace and heated at a temperature of 850 to 1100 F. for a period of two to sixty minutes for the purpose of fusing and maturing the slip or frit coating to the point where it is fused and forms a uniform coating which is bonded to the ferrous articles. Thereupon the COVER. CQAT lSAME PARTB WEIGHT Table. VIII... 10.0 a a Frit .(,A)., 10 Titania. ML, Refined bentonite. /2 Urea. 35 Water. 100 Frit.(B). 10 Titania..

1 i Coloring oxide. 3O .L Water. 4 Methanol. 100 Frit .(A).. 5.; Black.oxide.. /2; Refined. bentonite. A- .Urea. 30 Water- 5 Methanol.

The cover slips are made in-the sameway that the ground coatslips are made by grinding the frits shown in Table V with-the ingredients shown in the table and water with-methanol to formsuspensions to be used for. placing the coating on a metal sheet'having anunder coat. The sheet to which an under coat has been applied is suihciently clean .to directly receive. the cover coat and one or more coats may be applied to the under coat in. building up. a final enamel. coating of the desired thickness. 'It is prefer-red to apply a series of thin coatings one after. another in building up the-.desired enamel surface rather than to use a thickapplicationin an attempt to build up a thick enamel coating in a single operation.

It. will, beseen from Table II and V that the cornposi tions of. the. gro nd. an cover. oat va y omewha order to. o t inauniformbonded c tin h metu r ing mp ature sedissuflicie fly i hsq t at fu s 0f p ccurs nnni ngiha in a eta pr i n l t g o t eme l...

ature o he. p yarv n. bta z in flha flt. Soft. n med m ar wat n st, ev qm o ti file r n. ordqrt adapt. Q.. IK1. iP q i on d. i ter ntt mseratvrse. th n qatin i o. applied to a thin metal sheet it is important to have a slip. provide a finished composition which will have a thermal. linear. coefficient of expansion. within plus or minus 1 of the.coefficient, of expansion of, the steel to which itislto. be. applied. If this, relationshipexists between the.;coethcients. ofexpansionof the steel and the slip then the coated steelsheet maybe bentv or flexed repeatedly and the coating will be sufficiently flexible .so that it will not chip off when the steel is bent to. aco mparative'ly sharp angular condition.

The patents to Deyrup'2,338,099 and 2,467,114' disclose vitreous enamel frits which are applied to glass ware and aluminum at temperatures respectively of. 1150 and between 920 and 980 F'. The glass form: ing constituents of which these frits are made are in many respects the same as the constituents used in makingv the frits of the present invention; A large series oftests have been carried outwhen usingthe-frits of these patents at the temperatures specified in the' patents to learn whether these frits were suitable or could be used for applying a satisfactory vitreous enamel coating to steel. It was found that regardless of'thetemperatures used for maturing the frits on steel, or ferrous metals having a coetficientoflinear expansion below 14x10, that'satisfactory coatings could not-be obtained. In all 6 cases the fused 'ooatingschipped other the coatings contained so many fine and Iar ge 'checks that the coated. articles were. not :acid, alkali or. moisture. proof. The patents define critical relationships ofthe alkali metal oxides to the sum of silica and titania: contents of the frits which appearto apply only .to. glass and aluminum. These frits are not applicable to. ferrous alloys such as steel, cast iron, wrought iron and stainless steel, alloys which,have. a thermal coeflicient of linear expansion much lower than the'coefli'cient of linear expansion, of. aluminum andmuch'higher than the linear coeflicients of expansion of glassware.

Thevformulaeoutlihed in Tables I and II are critical. in obtaining thin flexible coatings that are firmly bonded to themmetal and mature into continuous completely covering v.cc'atingsJ The variations which are allowable,

are quite,srnall to provide for hard, soft and medium hard coatings and to provide for diflerent firing tempera: tures.

"In making up the frits the ingredients commonly used. are sodium, and potassium carbonates and nitrates, lithium carbonate, titanium dioxide, antimony trioxide, copper oxide, calcium. oxide, magnesia, zirconium oxide, lead oxides, dehydratediborax, calcium and sodium flu-.. orides, alumina, phosphorus ,pentoxide', barium oxide zinc" oxide, and cobaltoxide;v (C0 0 These ingredients. are-fusedtogetherinmaking the frits and the analyses: are exp'ress ed-as oxidesor as molecular fluorine in. the tables. g

Thetrade is now using;special low carbo1 1 steels for. the manufacture of iarticles. that are being enameled. at, temperatures above 1400 F. The. enamel frits of the. present. invention maybeeasily applied to any type of; steel, cast and wrought iron, and stainless steels whether the ferrous articles are thick or .thin. Thus theenamel and themethod of enameling of thepresent invention :are applicabl'ejto a large number of metal articles that could not be enameled heretofore.

The preferred form. of the invention having been thus described, what. is claimed as new is:

1. A lowtemperature porcelain enamel under coati'ng frit adapted to be bonded to ferrous metal alloys having a 'c'oeflicient of linear expansion below 14 10-, said frit having a fusing and maturing temperature be tween 850 and l10 0 R, which comprises a mixture expressed from analysis containing-mol. percentages of N320 to 33.4 to 38.7, CaO 1.0 to 15.6, and the balance of at least, one or more of the metahoxidesof the group consisting of Li O, CuO, MgO, TiO PbO, Al- ,O BaO, ZrO, P 0 NiO, C0 0 and fluorides.

.2. A lowitemperaturel orcelain enamel under coating frit 'as in claiml adapted to be bonded to ferrous metal; alloys,. having a coeflicient of linear expansion below 14x10, said frit having a fusing and maturing tempera v ture between 850 and 1100 F., and which comprises a mixture which expressed from. analysis contains metal oxides in the mol percent of the group consisting of Na O 13.9 017.3 K 0 0.4 to 3.0 Li 0.0 to 6.0: B 0 7.5 to 13.1' SiO' L 33.4 to 38.7 TiO i W i 0.0 to 4.2. 0.0 to 14.0 0.0 to 13.5

A1O 0.0 to 0.6 CuO 0.0 to 3.2

NiO 1.0 to 1.6 (30 0 0.0 to 0.2 ZrO l 0.0 to 3.0 P205;- Y 0.010 0.5 and.F;- 0.0 t0v 8 .3

3. A low temperature porcelain enamel frit adapted to cover coat ferrous metal alloys having a ground coat as defined in claim 2, said cover coat frit having a coefficient of linear expansion between 12 and 14 10- and a fusing and maturing temperature between 850 and 1100 R, which comprises a mixture expressed from analysis containing mol percentages of M 11.2 to 15.2

4. The low temperature porcelain enamel frit as defined in claim 2 to which a ceramic pigment in an amount from 0.5% to 10% is added to obtain desired colors.

5. A process of making a porcelain enamel ferrous metal alloy article having a coeificient of linear expansion in the range of 12 to l4 10- comprising removing grease, rust, scale and moisture from the surface to be coated, applying a uniform coating of a slip formed with a ground fused mixture of a frit as defined in claim 2 having a coeificient of linear expansion within :1 X 10 of the coeflicient of linear expansion of the ferrous metal alloy used, then heating the coated metal alloy article to a temperature in the range of 850 to 1150 F. for a suflicient period of 2 to 30 minutes to fuse and mature the slip enamel surface to a continuous bonded coating.

6. The process as defined in claim 5 wherein a cover coating of a slip formed from a frit as defined in claim 3 is applied, then heating the coated metal in the temperature range of 850 to 1100 F. to fuse and mature the cover coating to obtain a continuous bonded enamel surface.

7. The process as defined in claim 6 wherein the thickness of slip coating is between 1 to 3 mils of enamel thickness.

8. The process as defined in claim 6 wherein the thickness of enamel coating is within the range of 1 /2 to 15 grams per square foot of coated surface.

9. A low temperature porcelain enamel frit for bonding as under coat to ferrous metal alloys having a coefiicient of linear expansion between 12 10- and 14x 10- and for fusing and maturing as a continuous bonded surface at temperatures between 850 and 1100 F. comprising mol. percentages of 10. A low temperature porcelain enamel frit for bonding as an under coat to ferrous metal alloys having a co eflicient of linear expansion between 12x10? and 14 l0- and for fusing and maturing as a continuous bonded coating at temperatures between 850 and 1100 F. comprising mol. percentages of Na O 13.9 K O 1.9 L120- I H 5.5 B 0 11.7 $10 33.4 T10 2.8 PbO 13.0 Mgo 13.5 CaO 1.5 C 1.8 N10 1.0

11. A low temperature porcelain enamel frit for bonding as an undercoat to ferrous metal alloys having a coefficient of linear expansion between l2 10- and l4 10- and for fusing and maturing as a continuous bonded coating at temperatures between 850 and 1100 F. comprising mol. percentages of Na O 15.0 K 0 2.0 Li- O 6.0 B O 8.0 810; 36.0 T i0; 2.0, PbO 14.0 MgO 8.0 CaO 1.0 BaO 5.0 CuO 3.0

12. A low temperature porcelain enamel frit for bonding as an under coat to ferrous metal alloys having a coefiicient of linear expansion between 12x10" and 14 10- and for fusing and maturing as a continuous bonded coating at temperatures between 850 to 1100 F. comprising mol. percentages of Na O 16.3 K 0 3.0 L1 0 5.2 B 0 8.1 Si0 38.1 TiO 4.2 PbO 12.4 MgO 8.8 CaO 1.1 CuO 2.8

13 A low temperature porcelain enamel frit for bonding as an under coat to ferrous metal alloys having a coefficient of linear expansion between 12 l0- and 14 10- and for fusing and maturing as a continuous 14. A low temperature porcelain enamel frit for bonding as an under coat to ferrous metal alloys having a coefficient of linear expansion between 12 10- and 14 10- andfor fusing and maturing as a continuous 9' a bonded coating at temperatures between 850 and 1100' F. comprising mol. percentages of Na O 16.4 K20 0.4 A1203 0.6 B203 13.1 sio 35.1 F2 8.3 C3D 15.6 13210 5.2 ZrO 3.0 P205 0.5 NiO 1.6 C 04 0.2

15. A low temperature porcelain enamel frit for bonding as an under coat to ferrous metal alloys having a coeflicient of linear expansion between 12 10- and l4 10- and for fusing and maturing as a continuous bonded coating at temperatures between 850 and 1100 F. comprising mol. percentages of K 0 1.7 M 0 2.6 B 0 10.0 38.7 F 4.7 T103 2.1 PbO 6.0 CaO 9.0 BaO 3.0 ZrO 1.7 CuO 3.2

16. A low temperature porcelain enamel frit for use as a cover coat over an under coat bonded to the face of a ferrous alloy article, said frit having a fusing and maturing temperature between 850 F. to 1100 F. comprising mol. percentages as follows:

Na O 15.2 K 0 2.0 Li O 6.0 B 0 8.1 SiO 36.5 TiO 2.0 PbO 14.2 MgO 12.7 CaO 1.7 BaO 1.6

17. A low temperature porcelain enamel frit for use as a cover coat over an under coat bonded to the face of a ferrous alloy article, said frit having a fusing and matur- 10 ing temperature between 850 F. and 1100 F. comprising mol. percentages as follows:

Na O 15.2 K 0 2.0 Li O 6.0 B 0 8.2 Si0 36.5 Ti0 2.0 PbO 14.2 MgO 8.1 CaO 1.2 13210 6.6

18. A low temperature porcelain enamel frit for use as a cover coat over and under coat bonded to the face of a ferrous alloy article, said frit having a fusing and muturing temperature between 850 F. and 1100 F. comprising mol. percentages as follows:

K 0 2.0 Li O 6.0 B 0 8.1 S102 36.4 TiO 2.0 PbO 14.1 MgO 12.7 CaO 1.7 BaO 1.7 813 0 0.3

19. A low temperature porcelain enamel frit for use as a cover coat over an under coat bonded to the face of a ferrous alloy article, said frit having a fusing and maturing temperature between 850 F. and 1100 F. comprising mol. percentages as follows:

I Patent No. 2,842,458

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION July 8, 1958 Edward J, Feeney et alo It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, Table 1, first column thereof, line 9, for "PbO read P120 "same table and column, line 13, for "ZnO" read ZrO column 3, Table II, first column thereof, line 13, for "Z110" read ZrO same column, Table III, first column thereof, line 9, for "TiO" read TiO same table and column, line 10, for "P00 read PbO same column 3,

Table IV, fourth column thereof, under the heading "0'', lines 8 to 12, opposite "PbO" for "14.1" read 38.1 opposite "MgO" for "12.7" read 6.1 opposite "CaO" for "1.7 read 1.1 opposite "BaO'" for "1.7" read 2.9 opposite "Sb O for "0.3" read 1.0 *--3 column 5, line 51, g before "applied" insert be Signed and sealed this 4th day of November 1958.

(SEAL) Atte'st KARL -H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

1. A LOW TEMPERATURE PORCELAIN ENAMEL UNDER COATING FRIT ADAPTED TO BE BONDED TO FERROUS METAL ALLOYS HAVING A COEFFICIENT OF LINEAR EXPANSION BELOW 14X10**-6, SAID FRIT HAVING A FUSING AND MATURING TEMPERATURE BETWEEN 850* AND 1100*F., WHICH COMPRISES A MIXTURE EXPRESSED FROM ANALYSIS CONTAINING MOL. PERCENTAGES OF NA2O 13.9 TO 17.3, K2O 0.4 TO 3.0, B2O3 7.5 TO 13.1, SIO2 33.4 TO 38.7, CAO 1.0 TO 15.6, AND THE BALANCE OF AT LEAST ONE OR MORE OF THE METAL OXIDES OF THE GROUP CONSISTING OF LI2O, CUO, MGO, TIO2, PBO, AL2O3, BAO, ZRO, P2O5, NIO, CO3O4 AND FLUORIDES. 